The invention relates to color imaging and, more particularly, to calibration techniques for color imaging devices.
Calibration of an imaging device can significantly improve color accuracy of images rendered by the device. For example, imaging devices such as cathode ray tubes, liquid crystal displays, plasma displays, and various printing devices, are often calibrated to determine adjustments that can be applied to either color input or drive data applied to the device. In either case, the adjusted data can then be used to control the imaging device such that the ultimate rendition of the image has improved color accuracy. Calibration can be used to account for drift in the imaging device to improve color accuracy.
For example, the calibration of a cathode ray tube (CRT) may involve attaching a measurement device, such as a colorimeter, to the display screen to measure color output of the CRT. The measured output can then be compared to analytical expected color values to determine the color errors. The determined errors can then be used to modify a lookup table (LUT) in a video card associated with a host computer so that input color data can be converted in a manner that adjusts for the determined errors. The effectiveness and accuracy of the calibration process can substantially impact color accuracy.
Precise color accuracy is particularly important for color intensive applications such as soft proofing. Soft proofing refers to a proofing process that makes use of a display device rather than a printed hard copy. Traditionally, color proofing techniques have relied on hard copy proofing, where proofs are printed and inspected in order to ensure that the images and colors on the print media look visually correct. For instance, color characteristics can be adjusted and successive hard copy prints can be examined in a hard proofing process. After determining that a particular proof is acceptable, the color characteristics used to make the acceptable proof can be reused to mass-produce, e.g., on a printing press, large quantities of print media that look visually equivalent to the acceptable proof.
Soft proofing is desirable for many reasons. For instance, soft proofing can eliminate or reduce the need to print hard copies on media during the proofing process. Moreover, soft proofing may allow multiple proofing specialists to proof color images from remote locations simply by looking at display devices. With soft proofing, there is no need to print and deliver hard copy proofs to remote reviewers. Thus, soft proofing can be faster and more convenient than hard copy proofing. Moreover, soft proofing can reduce the cost of the proofing process. For these and other reasons, soft proofing is highly desirable. The ability to achieve precise calibration of soft proofing display devices is an important factor to achieving an effective soft proofing system.
In general, the invention is directed to various calibration techniques for calibrating an imaging device such as a display device, a printer or a scanner. The techniques may involve characterizing the imaging device with a device model, wherein an average error between an expected value of the device model and measured output of the image device is on the order of an expected error. The invention can achieve a balance between analytical behavior of the imaging device and measured output. In this manner, adjustments to image data may be more likely to improve color accuracy and less likely to overcompensate for errors that are expected.
In various embodiments, the invention may be directed to methods of calibrating an imaging device. For example, a method may include characterizing the imaging device with a device model such that an average error between expected outputs determined from the device model and measured outputs of the imaging device is on the order of an expected error. The method may also include adjusting image rendering on the imaging device to achieve a target behavior.
In another embodiment, the invention may be directed to a method that includes measuring outputs of the cathode ray tube for a subset of device values of the cathode ray tube, and choosing one or more parameter values of a device model, wherein the number of adjustable parameters is less than a number of measurements used to define the measured output of the cathode ray tube, and wherein an average error between expected outputs of the device model and the measured outputs is on the order of an expected error. The method may further include adjusting image data according to the device model to achieve a target behavior for the imaging device.
In another embodiment the invention may be directed to a method that includes initializing a lookup table (LUT), adjusting settings of the cathode ray tube to substantially achieve a defined output, and measuring output for a number of color values. The method may also include choosing parameter values for a device model, wherein a number of adjustable parameters is less than a number of measured outputs, and generating entries for the LUT based on the device model.
In another embodiment, the invention may implement a technique for biasing an output measurement by an amount sufficient to ensure that the output measurement is within a dynamic range of a measurement device. For example, a method may include measuring output of a display device, and displaying a substantially white trace during measurement to bias the output measurement. The trace may have a halo shape, or any other shape sufficient to properly bias the measurements.
In other embodiments, the invention is directed to calibrated imaging devices or sets of calibrated imaging devices. For example, in accordance with the invention, a cathode ray tube, or a set of cathode ray tubes can be calibrated such that an average color error is approximately less than (0.75 delta e) from an analytical expected color output, and a maximum color error is approximately less than (1.5 delta e) from the analytical expected color output. Furthermore, even more precise calibration, approaching a theoretical limits of analytic equations used to define device behavior can be achieved as described in greater detail below.
Various aspects of the invention may be implemented in hardware, software, firmware, or any combination thereof. If implemented in software, the invention may be directed to a computer readable medium carrying program code, that when executed, performs one or more of the methods described herein.
The invention is capable of providing a number of advantages. In particular, the invention can improve calibration of imaging devices. Moreover, improved calibration can facilitate the realization of color intensive applications such as soft proofing. In some cases, the invention can be used to calibrate imaging devices such that measured errors of the imaging device are on the order of expected errors. For example, expected errors in the measurements may be caused by factors unrelated to the imaging device, such as errors introduced by the measuring device or the video card. The invention can achieve a balance between theory and measurement to ensure that adjustments to image data do not overcompensate for measured errors unrelated to the imaging device itself. In this manner, an imaging device can be calibrated such that an average color error is approximately less than (0.75 delta e) from an analytical expected color output.